Steel can be produced through two different routes.
The first route comprises the production of pig iron inside a blast furnace and the transformation of this pig iron into crude steel in an oxygen converter or basic oxygen furnace (BOF). The second route consists in melting scraps in an electric arc furnace (EAF) to directly produce crude steel. After any of these routes, crude steel is then refined to obtain the required chemical composition of the steel, this refining step being performed in a ladle furnace (LF).
According to the first route, pig iron coming from blast furnace is poured into an oxygen converter, possibly comprising scraps. Oxygen is blown in the converter to allow decarburization of pig iron and its transformation in liquid steel. Mineral additives, such as lime and dolomite, are also added in the converter. The transformation of pig iron consists of fast oxidation reactions induced by the contact between gaseous oxygen and molten metal, in conditions which are very far from the thermodynamic equilibrium with the other present chemical elements, manganese, silicon or phosphorus for example. Such produced oxides, together with the added mineral additives, contribute to the formation of a liquid slag, which floats on the surface of the metal bath due to its lower density. For efficient purification of the metal, the equilibrium partition coefficients of the various elements (phosphorus, sulfur, etc.) between the slag and the metal should be as high as possible, corresponding, for example, to maximum values for the ratios: LP=% Pslag/% Psteel and LS=% Sslag/% Ssteel. The determination of the slag chemical composition allows producing quality crude steel.
According to the second route, metal scraps are loaded into a furnace and melted. The energy required to melt such solid scraps is mainly provided by electric arcs produced between one or several graphite electrodes and the metallic charge. The refining reactions are quite similar to those in the oxygen converter. Oxidation of the unwanted elements is obtained by oxidized impurities in the charge, by pure oxygen injected either through lances or through nozzles in the furnace, or by atmospheric oxygen which enters via furnace orifices. The oxidized impurities form the slag.
Crude steel produced by one of the previous routes is then poured into a ladle to adjust the chemical composition of the steel. The analytical quality of the liquid metal is adjusted, including compositional trimming, not only of metallic alloying elements, but also the control of metalloids (C, H, N, O, P, S), to different degrees depending on the grade. The type and content of oxide inclusions is controlled, by deoxidation (or “killing”) of the steel, generally with aluminum for sheet steels, by calcium treatment to modify their composition, and by controlled floatation. Different additives, such as lime, dolomite, fluorspar and/or various fluxes are added in the ladle furnace to perform such treatments.
As previously explained, the produced impurities form a slag floating on the surface of the molten metal. Depending on the composition of the slag, additives are added to remove remaining impurities. So the knowledge of slag composition is of primary importance in order to control the quality of the refined steel.
In the EAF process, the knowledge of the chemical composition of the slag allows knowing its basicity and oxidation. However, the chemical composition of slag is not known during the process. Samples are analyzed using spectrometers after the process is completed.
During the LF process, the degree of deoxidation and desulfuration of steel is also estimated based on the visual evaluation of slag samples and on the chemical composition of the steel. The visual appearance of the cooled down slag is still used during the process. Thus, adding ferro-alloys and other additives to the batch still has a human component due to the expertise and the subjectivity of the workers. To that end, spectroscopes are used after the process is completed. This requires additional time for the samples to be prepared.
In both cases this has an impact on the process control, with detrimental consequences on the amount of waste, productivity and production costs.
An aim of the invention is to provide a method of determining a chemical composition of a slag portion that solves or reduces at least some of the above mentioned issues, in particular that improves the productivity of the manufacturing process and remains easy to implement.